Weed of the Month: Fall Panicum

Published: September 1, 2011

Fall panicum (Panicum dichotomiflorum Michx.) is a summer annual grass weed found throughout the United States in a variety of agronomic, non-crop, nursery, landscape, and turfgrass settings. Over the past several years, fall panicum has become an increasingly problematic weed of corn and soybean production in Missouri.

Figure 1. Fall panicum seedlings have hairs on the leaf undersides only, but these disappear with maturity.

Fall panicum seedlings can appear much different than mature plants. Perhaps the most notable characteristic of fall panicum seedlings are the hairs that occur on the lower leaf surfaces (Figure 1). Seedlings also have hair-like ligules that typically measure 2 mm or less in length and are without auricles. Mature plants have leaves that are quite large at maturity, measuring 15-20 mm in width, are rolled in the bud, and are hairless on both surfaces. Leaves also have a distinct white midvein and are often glossy in appearance (Figures 2 and 3). With maturity, plants take on a "zigzagged" growth habit. Stems are relatively thick, hairless, and usually enlarged at the nodes. Fall panicum seedheads are wide, spreading panicles that develop a purplish tint when mature.

Fall panicum seed emergence occurs most readily at depths of 0 to 2 inches, with few seeds capable of germinating from soil depths greater than this. Other authors have observed extreme variability in the pattern of fall panicum emergence from one year to the next. However, most indications are that this species tends to be a later emerging weed that can germinate throughout the growing season.

In corn, atrazine usually will only provide fair pre-emergence, residual control of fall panicum, which may explain the higher incidence of this weed in our corn and soybean production systems over the past several years. Atrazine premixes that contain metolachlor (Bicep II Magnum, Charger Max, Lumax, Lexar, etc.), acetochlor (Degree Xtra, Harness Xtra, etc.), or dimethenamid (Guardsman Max) will increase control of fall panicum dramatically. Post-emergence control of fall panicum in corn is difficult and must be timely; applications must be made to small fall panicum plants in order to be effective. However, products that contain ALS-inhibiting herbicides like nicosulfuron (Accent), rimsulfuron (Resolve, Basis), thiencarbazone (Capreno) can provide good control of fall panicum as well as glyphosate (Roundup, etc.) and glufosinate (Ignite), in Roundup Ready and LibertyLink corn systems, respectively. Post-emergence applications of the HPPD-inhibiting herbicides Callisto, Impact, and Laudis generally only provide fair to poor control of fall panicum.

Figure 3. Mature fall panicum plants with seedheads.

Many of the pre-emergence, residual herbicides that are relied upon for the control of fall panicum in corn can be utilized in soybean as well. For example, pre-emergence applications of products that contain metolachlor (Dual II Magnum, Prefix, Boundary), alachlor (IntRRo), dimethenamid (Outlook, Verdict), and pendimethalin (Prowl, Prowl H2O) will provide good control of fall panicum in soybean. Post-emergence options for the control of fall panicum in soybeans include the grass herbicides like quizalofop (Assure II, Targa, etc.), fluazifop (Fusilade), and clethodim (Select Max, Arrow, etc.), glyphosate (Roundup, Touchdown, etc.) in Roundup Ready soybeans, and glufosinate (Ignite) in LibertyLink soybeans.